12.2.1 - Acid-base titrations, pH curves and indicators

Question 1

What is the end point of a titration?

Answer 1

The point at which the indicator used in the titration changes colour.

Question 2

What is the equivalence point of a titration?

Answer 2

When the acid and base have reacted together in exact proportions as dictated by the stoichiometric equation.

Question 3

For a strong acid-strong base reaction where the acid is being added, what does the pH start and end at?
What is the equivalence point?

Answer 3

Starts at 14
Ends at roughly 1

7

Question 4

For a weak acid-strong base reaction where the acid is being added, what does the pH start and end at?
What is the equivalence point?

Answer 4

Starts at 14
End at 3/4

8.5

Question 5

For a strong acid-weak base reaction where the acid is being added, what does the pH start and end at?
What is the equivalence point?

Answer 5

Starts at 11
Ends at about 1 or 0.5

5.5

Question 6

For a weak acid-weak base reaction where the acid is being added, what does the pH start and end at?
What is the equivalence point?

Answer 6

Starts at 11
End at 3/4

7

Question 7

What is significant about the graph of an weak acid-weak base raction?

Answer 7

There is no equivalence point, instead there is a point of inflexion

Question 8

What are most acid-base indicators?

Answer 8

Weak acids

Question 9

How do you know what the best indicator is for a reaction?

Answer 9

The one with PKin value closest to pH at equivalence point.

Question 10

What is the pH range of phenolphalein?

Answer 10

8.20 - 10.00

Question 11

What is the pH range of methyl orange?

Answer 11

3.10 - 4.40

Question 12

For the 4 different acid-base reactions describe which of the two indicators methyl orange and phenolphalein would work?

Answer 12

Strong acid - strong base = both
Weak acid - strong base = Only phenolphalein
Strong acid - weak base = Only methyl orange
Weak acid - Weak base = Neither

Question 13

We have a titration where sodium hydroxide is added to propanoic acid.
Why is their buffer action before equivalence point?

Answer 13

Here the solution resists changes to pH when alkali is added.
Both propanoic acid and propaonate ions are present.
CH3CH2COOH + OH- = CH3CH2COO- + H20